The present invention relates to a lithographic apparatus and a lithographic method, or more in particular to an apparatus and a method effective for forming a fine pattern on the order of submicrons.
If a fine pattern is to be formed, it is necessary to irradiate a lithographic beam on a silicon wafer with high accuracy. In order to irradiate a silicon wafer with high accuracy, the height of the water, i.e., the direction of beam irradiation must be determined correctly. The reason is described below.
When an electron beam machine is used as a lithographic apparatus, an electron beam is not irradiated in the direction perpendicular to the wafer. Unless the wafer is at a predetermined height, therefore, the beam irradiation is displaced from the designed position at the parts where the electron beam reaches with a great deflection, i.e., the peripheral parts of the drawing region. In an optical lithographic apparatus, on the other hand, the shallow depth of focus of ultraviolet ray causes an increased width of the beam irradiated on the wafer if the wafer is displaced, thereby often making it difficult to draw a fine pattern.
Methods and apparatuses for measuring the height of an object have conventionally been suggested.
The configuration described below, for example, is disclosed by JP(U)-A-52-139768, JP(U)-A-1-157423 and JP(U)-A-1-167733. Specifically, a laser beam is focused is spot and applied on an object, and the reflected beam is detected by a position sensor (a semiconductor sensor with an output signal changing depending on the beam-receiving position). The signal thus detected is amplified, subjected to an analog computation and converted into a voltage signal linearly changing with height, thereby measuring the height of the object.
The height information obtained in this way is used to correct the deflection of the electron beam and the focal point of the ultraviolet ray.
The conventional method described above poses no special problem in writing a reticle or mask pattern by an electron beam machine. In what is called the direct-writing process for drawing a pattern directly on a semiconductor wafer, however, the following problems are posed:
(1) In the direct-drawing process, a pattern is sometimes drawn over a circuit pattern already formed. When measuring the height in such a case, a signal of sufficient intensity cannot be detected resulting in a lower accuracy due to the scattering of the detected light by the base pattern.
(2) The beams reflected from the wafer surface and the resist (light-sensitive material) surface coated on the wafer interfere with each other and thus the detection signal is reduced leading to a lower accuracy.
For these reasons, a pattern drawing of high accuracy and density becomes difficult.
A height-measuring instrument so configured that the incident beam is applied at an angle of Brewster against the wafer is disclosed in JP-A-59-188931. This instrument includes a device for deflecting the reflected beam and receiving the deflected beam components.
JP-A-61-177719 is cited as a reference disclosing a configuration for detecting an alignment mark as one of the prior art related to the present invention. The art of correcting the electron beam on the basis of the difference between alignment mark height and pattern height is disclosed in JP-A-3-46220. According to this technique, the laser beam for detecting the height is irradiated only on a pattern region.
Further, JP-A-2-21553 is shown as another reference disclosing the art related to the present invention. This reference discloses the art for irradiating a laser beam for measuring the wafer position in the direction parallel to the lithographic beam.
Also, a measure to be taken against a tilted wafer is disclosed in JP-A-3-46725.
Further, JP-A-1-170022 discloses the art for measuring the heights of a plurality of points on the wafer.